1. Field of the Invention
This invention relates to a precise bearing for tape in a magnetic tape recorder. More particularly, the invention relates to a precise bearing providing a thin, dynamically-stiff air bearing or air film between the bearing surface and the tape.
2. Review of Problem and Prior Art Solutions
In high density magnetic tape recording, it is desirable to have a tape path supported in a precise position and supported with a dynamically-stiff air film. The stiff air film makes the tape path relatively invariant to small variations in tension in the tape. If an air film is spongy or soft, then as the tape moves around a circular bearing, changes in tape tension would change the thickness of the air film and thus the tape path. As tape tension increases, the radius of the tape path would decrease. A dynamically-stiff air film, on the other hand, does not materially change its thickness with variations in tape tension.
To obtain a dynamically-stiff air film, the air film should be thin and rapidly change in pressure as tension in the tape causes the tape to press down on the film. This can be accomplished by using small holes to conduct air flow from a pressure chamber inside the bearing to the surface of the bearing. In addition, the pressure in the plenum inside the bearing should be much higher than the maximum pressure required to keep the tape off the surface of the bearing. In such a bearing structure the small holes providing the air to the surface of the bearing will keep the air flow small and provide large pressure drops that change rapidly with changes in air flow.
The rapid change in pressure can be seen by examining the effect on a circular bearing if the tension in the tape goes up causing the tape to push down on the air film. As the tape lowers in flying height over the bearing surface, the air flow decreases rapidly. As the air flow through the holes in the bearing surface decreases rapidly, the pressure in the air film rapidly increases closer to the pressure in the plenum supply. Thus a small change in flying height due to tension in the tape brings a rapid change in film pressure to balance the force pushing the tape down on the film. This rapid dynamic adjustment can be referred to as dynamic stiffness in the air film of the bearing.
To obtain a bearing having a dynamically-stiff air film characteristic, there are two known ways of constructing the bearing. One bearing uses a porous material. The small pores in the material serve as the air holes or passages for air under pressure inside the bearing to the film on the outside of the bearing. Materials that have been used include porous ceramics or porous metals such as brass or stainless steel. A difficulty with such bearings is that a precise-surface contour cannot be obtained with these bearings. To obtain a precise-surface requires a grinding operation or some other type of surface finishing operation. Trying to grind or surface finish a porous surface results in filling in the pores in the surface and thus destroying its porosity.
An alternate solution to obtaining a bearing surface with a precise contour, and having a capacity to generate a thin, dynamically-stiff air film, would be to use rigid material which could be ground to a fine precision-surface and thereafter drill small holes in the surface. However, putting small holes in a rigid thick material is not easily accomplished. As a practical matter for holes less than 0.025 inch (0.635 mm.) in diameter, a rule of thumb is that the small hole cannot be drilled any deeper than the diameter of the hole. Thus a hole with a 0.010 inch (0.254 mm.) diameter as a practical limit can only be placed in sheet stock 0.010 inch (0.254 mm.) thick. Accordingly, it is not practical to drill holes 0.010 inch (0.254 mm.) in diameter through thick stock material, for example 0.125 inch (3.175 mm.) thick.
The next possibility for fabricating a precise-bearing surface with small holes is to precisely machine a thick rigid bearing support with air channels therein. Then overlay the bearing with a thin foil having the proper size small holes to produce the thin stiff air film. The holes of course would be aligned with the air channels in the bearing support. A bearing constructed in this way would consist of rigid stock material approximately 0.125 inch (3.175 mm.) thick. The stock material would be ground to the precise contour desired, and channels would then be machined into its surface to provide an air pressure chamber underneath a foil. The foil with the small holes etched in it would then be bonded to the surface of the bearing. The foil would typically be 0.005 inch (0.127 mm.) thick and would have holes in it approximately 0.005 inch (0.127 mm.) in diameter.
The problem with such a structure is that the outside contour of the bearing cannot be precisely controlled because of the very thin nature of the foil. The thin foil is so flexible that when bonded to the rigid support bearing, especially in a heat bonding operation, the foil will dimple where the air channels are beneath the foil. Further, precise grinding of the foil thereafter is impossible because the foil is so thin. Thus, practically speaking, the concept of bonding a thin foil having small holes to a rigid precise bearing is not feasible because of the flexibility of the foil prevents the bearing from having a precise contour.
It is the object of this invention to provide a very precise rigid bearing having the capability of producing a thin air film with high dynamic stiffness so that magnetic tape passing over the bearing will have a very precise and dynamically stable tape path.